This invention relates to all-gear differentials of the type commonly referred to as xe2x80x9climited-slipxe2x80x9d designed primarily for automotive use and, more particularly, to such differentials designed for use in vehicles where efficiency, space, cost, and weight are critical.
The general format and operation of the type of differential being improved by the present invention is well known, and this prior art type of differential has enjoyed fairly widespread use and publicity throughout the world under the trademark xe2x80x9cTORSEN(copyright)xe2x80x9d.
As is shown in FIG. 1, a prior art limited-slip differential of the type just mentioned above includes a rotatable gear housing 10, and a pair of drive axles 11, 12 are received in bores formed in the sides of housing 10. A flange 13 is formed at one end of housing 10 for mounting a ring gear (not shown) for providing rotational power from an external power source, e.g., from a vehicle""s engine. The prior art gear arrangement within housing 10 is often called a xe2x80x9ccrossed-axis compound planetary gear complexxe2x80x9d and includes (a) a pair of side-gear worms 14, 15 fixed, respectively, to the inner ends of axles 11, 12, and (b) several sets of combination gears 16 organized in pairs, each combination gear having outer ends formed with integral spur gear portions 17 spaced apart from a xe2x80x9cworm-wheelxe2x80x9d portion 18. [NOTE: While standard gear nomenclature uses the term xe2x80x9cworm-gearxe2x80x9d to describe the mate to a xe2x80x9cworm,xe2x80x9d this often becomes confusing when describing the various gearing of an all-gear differential. Therefore, as used herein, the mate to a worm is called a xe2x80x9cworm-wheelxe2x80x9d.]
Each pair of combination gears 16 is mounted within slots or bores formed in the main body of housing 10 so that each combination gear rotates on an axis that is substantially perpendicular to the axis of rotation of side-gear worms 14, 15. In order to facilitate assembly, each combination gear 16 usually has a full-length axial hole through which is received a respective mounting shaft 19 for rotational support within journals formed in housing 10. [NOTE A few known prior art combination gears were formed with integral hubs that are received into the journals of housing 10; but to facilitate design of the housing and assembly, the combination gears of most presently used limited-slip differentials of this type are shaft mounted.] The spur gear portions 17 of the combination gears 16 of each pair are in mesh with each other, while the worm-wheel portions 18 are, respectively, in mesh with one of the side-gear worms 14, 15 for transferring and dividing torque between axle ends 11, 12. In order to carry most automotive loads, prior art differentials of this type usually include three sets of paired combination gears positioned at approximately 120xc2x0 intervals about the periphery of each side-gear worm 14, 15.
While the conventional full-length axial hole through each combination gear 16 (for receiving mounting shaft 19) facilitates assembly, it creates definite limitations and problems regarding the manufacture of combination gear 16. First, and most obvious, combination gear 16 is often measurably weakened by the full-length axial hole. Also, the hole places a limitation on the depth of the gear teeth formed on the cylindrical surfaces of the gear, particularly on the depth of the teeth on worm-wheel portion 18. Further, during necessary heat treatment for hardening the teeth formed on the cylindrical surfaces of combination gear 16, the full-length axial hole for receiving mounting shaft 19 must often be xe2x80x9cmaskedxe2x80x9d to reduce potential heat distortions of the teeth caused by the relatively thin wall that is created at the bottom lands of the worm-wheel teeth.
The tooth contact patterns of most known worm/worm-wheel combinations are conventional xe2x80x9csingle-envelopingxe2x80x9d patterns. [NOTE: While not appropriate for this use, certain machine tool worm/worm-wheel combinations have xe2x80x9cdouble-envelopingxe2x80x9d tooth contact patterns. However, these require the use of hourglass worms mated with fully conjugate worm-wheels of greater diameter, while the full-traction differentials of the invention, as will be explained below, are designed with cylindrical worms mated with hourglass worm-wheels of smaller diameter.] Single-enveloping worm/worm-wheel combinations are known to mesh with a xe2x80x9cline contactxe2x80x9d in a relative xe2x80x9cscrew-likexe2x80x9d motion (as different from the rolling engagement of spur or helical teeth), and the same line contact pattern is shared by the side-gear worms and the worm-wheel portions of the combination gears in prior art crossed-axis limited-slip differentials. Line contact, particularly when experienced in the relative xe2x80x9cscrewingxe2x80x9d motion of worm gearing, tends to squeeze lubricant off tooth surfaces and, therefore, necessitates the use of fairly viscous lubricants to prevent galling.
This type of prior art limited-slip differential also has another disadvantage. An undesirable torque imbalance occurs whenever the vehicle""s engine is applying torque to the differential. This results from the fact that the helical side-gear worms have the same helix angle and are, in effect, in sliding contact with each other at all times so that, when under load, both are subject to thrust forces in the same direction (e.g., both are simultaneously thrust to the left when the vehicle is being driven forward or simultaneously thrust to the right when the vehicle is driven in reverse). For instance, when subjected with thrust force X directed to the left, the right worm presses against the left worm with the force of X, while the left worm is pressed against its respective thrust bearing with a force of 2X. Since the differential divides the torque between the two axles, and since the torque distribution is affected by the friction overcome in each leg of the division, the just-described imbalance affects the torque bias of the differential. The following exaggeration may help to explain the effects of this imbalance: If the torque bias of the differential is designed for 8-to-1 (i.e., eight times more torque being directed to the slower moving wheel), when turning in one direction, the just-described imbalance might result in a torque bias of 10-to-1; and while turning in the opposite direction, the just-described imbalance might result in a torque bias of 6-to-1. While the prior art uses washers and bearings with low-friction surfaces to reduce the imbalance, it would be preferable for vehicle handling and maneuverability to totally avoid such thrust duplication between the side-gear worms.
Nonetheless, in actual practice, this type of prior art differential does a remarkable job of preventing undesirable wheel slip under most conditions. In fact, one or more of these limited-slip differentials are either standard or optional on vehicles presently being sold by at least eight major automobile companies throughout the world, and there are two of these TORSEN(copyright) limited-slip differentials in every U.S. Army HMMWV (xe2x80x9cHummerxe2x80x9d) vehicle (one differentiating between the front wheels and the other between the rear wheels). Nonetheless, these differentials are relatively large and heavy, taking up valuable space in the mechanically crowded modern vehicle; and they add fuel-consuming extra pounds to the vehicle""s weight.
The invention herein is a significantly more compact xe2x80x9cfull-tractionxe2x80x9d differential that avoids the thrust duplication between the side-gear worms, is smaller in both size and weight, and is less costly to manufacture while meeting similar load-carrying specifications. [Prior art TORSEN differentials are presently commonly referred to as xe2x80x9climited-slipxe2x80x9d, and almost all of the TORSEN differentials presently being manufactured and sold are designed with relatively low torque bias ratios, no greater than 5-to-1. While the invention disclosed herein can be designed with torque bias in that same range, it is preferably designed for torque bias ratios greater than 5-to-1. Therefore, the term xe2x80x9cfull-tractionxe2x80x9d is used herein to distinguish the differentials of the invention.]
A cartridge-like gear complex is disclosed for use in a full-traction differential, and it differs from the prior art in several interrelated structural and design features. Each feature by itself provides some improvement inefficiency and/or some reduction in size and weight when compared with prior art limited-slip differentials having similar load-carrying specifications. However, when these several interrelated features are combined in a single differential, there is a synergistic effect that meets automotive specifications with greater efficiency, with significant weight reduction, and in a remarkably compact housing.
While these various synergistic features are all related, understanding is facilitated by introducing and explaining them individually.
The first feature is a basic change in the format of the combination gears and their respective mountings. In place of either the prior art""s through hole for receiving a journal pin or the earlier prior art integral hub at each end, each combination gear of the invention has only a relatively shallow journal hole in each end of an otherwise solid gear body. (Note: As used herein, the term xe2x80x9cshallow holexe2x80x9d refers to a hole that has a depth that is, at most, only a little larger than its diameter.) A mounting for supporting the sets of paired combination gears is provided with a plurality of sets of paired hubs fixed to the mounting and positioned, respectively, to extend axially into the shallow journal holes formed in the ends of each combination gear. By itself, the incorporation of this mounting arrangement reduces the outer diameter and overall weight of the differential without requiring any change in the load-carrying specifications for either the spur gear teeth or the worm-wheel teeth of the combination gears or the worm teeth of the side gears.
The just-described format change of this first feature also reduces manufacturing costs by (a) avoiding the manufacture of conventional independent journal shafts (that must be heat treated and ground) as well as the drilling of accurate holes within the housing for both the journal shafts and their necessary lock pins, and by (b) making the combination gears without the time and expense of masking. This first feature also simplifies assembly by avoiding the time and costs that would otherwise be required for the manual assembly of the multiple parts necessary for supporting each conventional combination gear.
The solid central portion of each combination gear, provided by the just-described first feature, leads to a further size and weight reduction feature. Namely, the solid central worm-wheel portion of each combination gear can be designed with a deeper hour-glass shape (e.g., by using a hob having the same outside and pitch diameters as the side gear) without jeopardizing strength.
A second feature of the invention comprises a different tooth design for the differential""s worm/worm-wheel gearing. The tooth surfaces of the worm/worm-wheel gearing in known limited-slip differentials are involute in shape, being cut with straight-sided hob teeth and, as indicated above, have line contact. However, according to this second feature, only the worm-wheel portions of the combination gears retain this involute shape, while the side gears are cut with involute cutters and thus have an xe2x80x9cinverse-involutexe2x80x9d shape. In contrast to the line contact of the prior art, the respective involute and inverse-involute surfaces according to the invention mate with each other in a relatively broad xe2x80x9csupra-envelopingxe2x80x9d contact pattern on only the drive side of the mesh.
This just-described supra-enveloping contact pattern spreads the load over such a significantly wider area that it is possible to use only two pairs of combination gears (spaced, respectively, at 180xc2x0 intervals) rather than the more conventional three pairs (spaced, respectively, at 120xc2x0 intervals) to carry a given load. That is, this improved tooth design creates greater areas of tooth engagement as well as increasing the number of teeth in contact at any given time, making it possible to meet automotive specifications with two fewer gears. [Of course, this same new gearing can make it possible to carry significantly greater loads with the conventional three pairs of combination gears, and separate embodiments of the invention are disclosed for such three-pair designs.]
Also, as different from conventional line contact that concentrates the load, the supra-enveloping contact of this new gearing spreads the load over a relatively larger area and results in less shearing of the lubricating oil film, thereby permitting the use of lower viscosity lubricants and assuring longer part life.
A further note regarding the preferred design of the invention""s worm/worm-wheel teeth: A salient feature of the crossed-axis gear complex of high-traction differentials is the mechanical advantage resulting from the worm/worm-wheel combination in the gear train between the vehicle""s wheels and the differential. As a vehicle travels around curves, the weight and inertia of the vehicle cause the wheels to roll simultaneously over the surface of the road at varying speeds, resulting in the need for differentiation. The initiation of such differentiation is greatly enhanced by a mechanical advantage between the side-gear worms and their mating worm-wheels. Of course, this same gearing results in mechanical disadvantage when torque is being transferred in the opposite direction. The preferred embodiments of the invention select 35xc2x0/55xc2x0 for the worm/worm-wheel teeth to provide both full traction as well as relative ease of differentiation, a selection that also makes the invention particularly appropriate for vehicles including ABS (automatic braking systems) having traction controls.
In another feature, the invention""s different tooth design also includes side-gear worms having xe2x80x9cboxedxe2x80x9d (closed-end) teeth. [NOTE: Ample working depth is provided by cutting teeth using face hobs having a reduced diameter.] The boxed tooth ends permit the side-gear worm teeth to be designed with greater working depth, thereby also permitting a reduction in the diameter of the side-gear worms without loss of load-carrying ability. At the same time, the supra-enveloping contact permits the axial length of the side gears to be smaller, carrying the same loads in spite of their reduced size. In a preferred embodiment of the invention, both the diameter and axial length of the cylindrical side-gear worms are considerably reduced by cutting the worm teeth with a hob having approximately the same relatively small diameter as the worm-wheel portion of the combination gears, resulting in deeper boxed teeth on the side-gear worm.
A still further feature provides torque balancing that equalizes the end thrust on the respective side-gear worms during vehicle turning, when being driven in either forward or reverse, regardless of the direction of travel. A thrust plate is supported by the same mounting that supports the sets of paired combination gears, being fixed against lateral movement and maintained between the inner ends of the side-gear worms. Thus, when under thrust to the left, the right worm exerts a thrust force X against the thrust plate, and the left worm exerts only its own thrust force X against the housing rather than the 2X force as explained above in the Background portion of this specification. Similarly, when under thrust to the right, the left worm exerts a thrust force X against the thrust plate, and the right worm exerts only its own thrust force X against the housing. In one disclosed embodiment, the thrust plate is also used to pilot the side-gear worms and, thus, the inner ends of the axles.
As suggested above, the just-described structural and design features, when combined, have several synergistic effects, including: (1) For a given load, the number of combination gear pairs can be reduced from three to two. (2) The housing, including the two bearing portions that receive the axle journals, can not only be made smaller but can be formed in one piece. (3) A single additional rectangular hole is formed through the one-piece housing, permitting assembly without the additional cost of bolting on a separate cap. (4) The mounting for the combination gears, as just described above, can be readily fitted within the rectangular hole in the housing, being piloted and supported by the side-gear worms and axles. (5) The housing can be made with an appreciably smaller diameter without reducing the size (and strength) of the load-bearing segments connecting with the ring gear mount. (6) Further, it should be noted that the preferred embodiment just referred to above (with stronger and deeper teeth and the reduction of both the diameter and axial length of the cylindrical side-gear worms) results in additional reductions in the length as well as the diameter of the housing. (7) Finally, the efficiency of the differential is improved by the thrust plate supported by the same mounting as the combination gears.
The automobile industry seeks constant product improvement and places a premium on cost, weight, and size reduction. In addition, we live in a world with a dwindling supply of raw materials which must be conserved. The invention herein serves all of these needs by providing drivers with a full-traction differential that facilitates safe automotive handling in a more compact, more efficient, and less expensive format without sacrificing strength or durability.